Communication receiver with noise blanking



y 7 1954 R. T. MYERS ETAL 3,140,446

COMMUNICATION RECEIVER WITH NOISE BLANKING Filed Aug. 3, 1962 A LOCAL 6 FlG.l 4 oscILLAToR T0 ,3 l 7; REMAINING R.I=. DELAY R.F. IST b LF. STAo%ES MIXER AMPLIFIER AMPLIFIER LINE AMPLIFIER l RECEWER o f 5 TUNEDRF PULSE AMPLIFIER DETECTOR SWITCH i CONTROL 1? Q INVENTORS'.

7 RICHARD T. MYERS,

FRED. E.SPANGLER,

BY )QMWIQ THEIR ATTORNEY? United States Pat'ent O 3,140,446 COMMUNICATION RECEIVER WITH NUISE BLANKING Richard T. Myers and Fred E. Spangler, Lynchburg, Va.,

assignors to General Electric Company, a corporation of New York Filed Aug. 3, 1962, Ser. No. 214,650 9 Claims. (Cl. 325-478) This invention relates to a communication receiver of the type which includes a separate circuit for selectively interrupting signal transmission in order to cancel or blank impulse noise. More particularly, it relates to such a receiver wherein further provision is made for disabling the noise blanking circuit whenever conditions are such as to produce excessive blanking of the receiver.

The problem of impulse noise has posed a'great many difficulties in the radio communication field. This type of noise generally consists of impulses randomly spaced in time, having very random amplitudes and are generally and characteristically produced by electrical arc discharge phenomena either man-made or natural. Typical of the man-made disturbances which produce impulse noise are electrical arc discharges produced by ignition systems of modern day automobiles and at the brushes or commutators of rotating electrical machinery. Ignition impulse noise, for example, is characteristically constituted of pulses of very short duration, in the order of fractions of microseconds, having an extremely fast rise time. These steep fronted impulses in passing through the highly selective tuned circuits of the receiver, particularly those deep within the receiver, such as the intermediate frequency (I.F.) stages, are stretched substantially so that at the output of the receiver, the noise impulses which initially were only fractions of a microsecond in duration have been stretched to several hundred microseconds. As a result, various noise problems are present which interfere substantially with proper reception.

This problem of noise, and particularly of impulse noise, has been the subject of concern to radio engineers almost since the very beginnings of radio communication. Many solutions to this problem have been proposed. As far back as the mid-thirties, J. J. Lamb described one solution in U.S. Patent No. 2,101,549, issued December 7, 1937, entitled Silencing Circuits for Radio Receivers and in a corresponding article entitled A Noise Silencing LF. Circuit for Super-Heterodyne Receivers published on page 11 of the February 13, 1936, issue of Q.S.T. Lamb suggested a scheme in which a separate noise pulse detecting and blanking channel is provided within the receiver to interrupt the signal transmission in the receiver upon occurrence of a noise impulse thereby preventing the noise from passing through the receiver to the speaker or any other similar uti1ization circuit. In the Lamb patent and article, and in subsequent and more recent articles, such as, for example, the article published by W. F. Chow entitled Impulse Noise Reduction Circuit for Communication Receivers, Transactions-I.R.E., PGVC Volume VC9 No. 1, May 1960, pp. 1 to 9, this function is achieved by detecting the noise pulses superimposed on the received intelligence bearing carrier in a suitable pulse detector and utilizing the detected pulse to control signal transmission in the receiver by blanking or cutting off one or more of the stages of the receiver.

While receivers utilizing such noise blankers have been found satisfactory for many purposes, they do suffer from a number of shortcomings which severely limit their usefulness to modern day operation. One of the most troublesome of these limitations stems from the fact that Ice in some circumstances the noise blanker may interrupt the receiver so often that signal reception in the receiver is seriously degraded. The nature and magnitude of this problem can be understood more clearly in terms of the following example: As pointed out above, at the front end of the receiver, particularly at the antenna, the duration of the noise impulses is extremely small, anywhere from a fraction of a microsecond to a full microsecond. By the time these noise pulses have passed through the selective tuned circuits of the radio frequency (RF) amplifying stages of the receiver, the pulses have been stretched and may have a duration of several microseconds. In order to blank these noise pulses properly, even at the front end of the receiver, a blanking pulse having a duration of the order of ten microseconds is customarily used to interrupt the transmission of the signal through the receiver. It is clear, therefore, that if the repetition rate of the noise impulses is a hundred thousand (100,000) per second, for example, and a ten microsecond blanking pulse is used, the blanking circuit is operative of the time and the receiver is effectively shut off. Even with a repetition rate of 50,000 per second, the receiver is shut off 50% of the time, which corresponds to a 3 db loss in the receiver. At lower repetition rates, the receiver is shut off for correspondingly lesser but still substantial periods of time. Some arrangement is desirable, therefore, which automatically disables the noise blanking circuitry whenever the repetition rate is sufficiently high to degrade signal reception.

It is, therefore, one of the principal objectives of this invention to provide a receiver having noise blanking circuitry for preventing the passing of impulse noise and control circuitry for disabling the blanker automatically if the repetition rate of the impulse noises is sufficiently high as to degrade signal reception.

Although the repetition rate of random noise impulses, such as ignition impulses, may from time to time present difiiculties in causing loss of signal intelligence due to excessive blanking of the receiver, a much more troublesome and difficult problem is due to intermodulation. That is, although the repetition rate of ignition noise impulses may be excessive for a short period of time; because of the random nature of these ignition noise impulses it is highly unlikely that the undesirably high repetition rate would continue on a steady state basis. Intermodulation, however (which may be defined as the resultant or beat frequency generated whenever two signals of dilferent frequencies are simultaneously impressed on a nonlinear device such as the pulse detector in the noise blanking channel) is a much more troublesome and difficult problem since it can result in steady state blanking of the receiver at an excessive rate. That is, the product frequencies thus produced are usually the sum and difference frequencies of the two signals and can produce blanking of the receiver at a continuing and high repetition rate. The nature of the problem may be understood more clearly by consideration of the following: The Federal Communication Commission (FCC) has assigned the following frequency bands for use in mobile radio service: 25-50; 150.8-174; and 450-470 megacycles. Within these frequency bands, individual channel allocations are on the basis of 20 kc. spacing in the 25-50 mc.

band, 30 kc. spacing in the 150.8474 mc. band, and 50 kc. spacing in the 450470 mc. band. It is apparent, therefore, that there are a great number of closely spaced channels in the mobile radio service. In a typical mobile F.M. receiver, the selective circuits at the front end of ceiver and in the noise blanking channel, are capable of receiving a great number of individual channels having either 20, 30 or 50 kc. spacing. These numerous interfering channels when applied to the nonlinear detector in the blanking channel, can produce difference or beat frequencies in the range from twenty to several hundred kilocycles. These beat frequencies due to interfering adjacent channels are capable of blanking the receiver at this beat frequency. It can be seen therefore that intermodulation in the blanking channel may seriously degrade signal reception in the receiver due to excessive blanking.

It is, therefore, a further objective of this invention to provide a receiver having a noise blanking circuit arrangement which is made insensitive to intermodulation problems by selectively disabling the noise blanker;

Yet another objective of this invention is to provide a receiver having a noise blanking arrangement which is automatically disabled in the event that steady state intermodulation conditions cause excessive interruption of the receiver;

A still further objective of this invention is to provide a receiver including a noise blanking arrangement wherein the noise blanker is disabled automatically whenever the noise impulse repetition rate is sufiiciently high to degrade the receiver performance either on a short term basis or continuously;

Other objects and advantages of the instant invention will become apparent as the description thereof proceeds.

In accordance with the invention, the foregoing objectives are achieved by providing a communication receiver which includes an auxiliary noise blanking circuit wherein the noise impulses are amplified, detected and converted to a blanking pulse that is applied to the signal transmission path of a receiver to bias one of the receiver stages into the nonconducting condition to prevent passage of the noise impulses. In addition, circuitry is provided in the blanker channel for generating a control signal proportional to the repetition rate of the blanking pulses and a feedback loop for conrtolling the noise blanker in response to the control signal. Whenever the repetition rate of the pulses, due to ignition noise or an intermodulation beat frequency, increases beyond a predetermined value, the control signal magnitude becomes sufficiently large to disable the blanking circuit thereby terminating further interruption of the receiver signal transmission path.

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a block diagram of the front end of a communication receiver constructed in accordance with the invention which includes a noise blanking channel and a blanking channel disabling circuit;

FIG. 2 is a circuit diagram of a portion of a transistorized noise blanking circuit incorporating the arrangement for disabling the blanking circuit; and

FIG. 3 is a tubed version of the circuit arrangement illustrated in FIG. 2.

FIG. 1 illustrates in block diagram form the front end of a typical double conversion, super-heterodyne communication receiver constructed in accordance with the invention and which includes a noise blanking channel and an arrangement for automatically disabling the noise blanking channel. The signal at receiver antenna 1 is amplified in one or more radio frequency (R.F.) amplifier stages illustrated at 2. The amplified signal may, if desired, be passed through a delay line 3 in order to synchronize the noise impulses in the signal transmission channel and the noise blanking pulses. The signal is further amplified in one or more radio frequency (R.F.) amplifying stages 4, and is impressed on the input of a first mixer arrangement 5 along with a signal from a local oscillator 6. The amplified signal is converted in mixer 5 to a lower or first intermediate frequency which may, for example, be 8 me. The intermediate frequency (I.F.) signal is then applied to one or more I.F. amplifier stages 7 and from there to the remaining stages of the receiver, not shown, which may typically include a second mixer to reduce the frequency of the signal to a second I.F. frequency, (such as 455 kc.) further I.F. amplifiers, and thence to a detector and the audio output stages of the receiver. It will be apparent, as the description of this invention proceeds, that the novel noise blanker and noise blanker disabling circuit may be utilized with communication receivers of many types and is by no means limited to a superheterodyne, double conversion receiver.

As was pointed out previously, various types of noise impulses, such as those generated by the ignition systems of automobiles, must be prevented from passing through the receiver since the noise impulses are, in passing through the highly selective tuned circuits of the 1.1 stages, stretched substantially and appear as interfering noise sound output of substantial duration. To this end, the received signal at the antenna 1 is also applied to the noise blanking channel, shown generally at 8, wherein the noise pulses are detected and utilized to provide a control or blanking pulse which disables one or more stages in the radio frequency amplifier 4 of the main signal transmission path of the receiver at the proper instant thereby preventing passage of the noise pulse. The noise blanking channel contains one or more tuned RF. amplifier stages, shown generally at 9, wherein the signal and the noise pulses received at the antenna are amplified sufiiciently to assure easy detection thereof in suitable pulse detecting circuitry illustrated generally at 10.

Pulse detector 10 may be any suitable well known circuit for detecting the noise impulses superimposed upon the received carrier. For example, in the case of an FM. receiver, pulse detector 10 may typically be an envelope detector comprising the combination of a semiconductor diode and a filter or bypass capacitor for detecting any sudden amplitude variations in the carrier and converting them into positive pulses which appear at the output of pulse detector 10. The positive detected pulses are impressed on a noise blanking switch 11 to generate blanking pulses of suitable polarity which are applied to one or more of the radio frequency amplifier stages 4 to bias these stages into the nonconducting state thereby interrupting signal transmission and preventing passage of the noise impulses to the remaining portions of the receiver.

Blanking pulse switch 11 may consist of any suitable combination of circuitry to produce a blanking pulse of the proper polarity, duration and amplitude to bias one of the RF. amplifier stages into nonconduction. In the specific switch circuitry to be described below, the switch consists of a pulse amplifier and a mono-stable or oneshot multivibrator. The one-shot multivibrator produces a blanking pulse of fixed duration and amplitude, and of the proper polarity in response to each incoming detected pulse from pulse detector 10. However, it will be obvious to those skilled in the art that many other circuit configurations may be utilized to produce the blanking pulses. For example, blanking switch 11 may consist of a pulse amplifier and suitable pulse limiting and shaping circuitry to produce the blanking pulse of the desired polarity, amplitude and duration. It is to be understood, therefore, that the instant invention is not limited to any particular circuitry for producing the blanking pulse.

As was pointed out in the previous discussion, it is necessary to prevent excessive blanking of the receiver by providing circuitry for disabling the noise blanker channel in the event that the noise impulse repetition rate, whether due to a high incidence of ignition impulses, or due to intermodulation problems, exceeds a predetermined level. To this end, a blanking switch control circuit in the form of feedback loop 12 is coupled between the output and input of noise blanking switch 11. Feedback loop 12 includes means for producing a control voltage in response to the blanking pulses of the proper polarity to disable switch 11 and the amplitude which is proportional to the repetition rate of the blanking pulses. Whenever the repetition rate of the pulses increases beyond a predetermined value, the amplitude of the control signal becomes sufliciently large to disable the blanking switch and thereby terminating further interruption of the receiver signal transmission path. The precise characteristics and circuit configuration of the switch control feedback loop will be explained in detail below in connection with FIGS. 2 and 3 of the drawings. However, it will be understood that any type of circuitry which produces a control signal proportional to the repetition rate of the blanking pulses is suitable for use in the novel communication receiver embodying the instant invention.

FIG. 2 illustrates a preferred form of the blanking switch and blanking switch feedback control circuit which may be utilized in the noise blanking channel of the receiver illustrated in FIG. 1. The positive, detected pulses 13 from the pulse detector are impressed across a pair of input terminals 14 and 15 and are coupled through coupling capacitor 16 and a diode 17 to the base of an NPN transistor 18 which forms part of a pulse amplifying circuit 19. Transistor 18 includes base 20, collector 21 and an emitter 22. Collector 21 is connected to positive bus 23 of a suitable source of energizing voltage through collector resistance 24, and emitter 22 is connected through an emitter resistance 25 to negative bus 26 of the source of energizing voltage. Emitter resistance 25 is bypassed for AG. by a suitable capacitor and together with a voltage dividing network, shown generally at 28, establishes the biasing conditions for amplifier 19. 7 Voltage dividing network 28 normally biases pulse amplifier 19 for class AB operation; that is, transistor 18 is normally biased to be slightly conducting. Network 28 consists of the voltage dividing resistances 29 and 30 connected between positive bus 23 and negative bus 26 through diode 17, with diode 17 connected between the junction of these two resistors. The cathode of diode 17 is connected to the upper end of resistance 30 and the anode connected to the lower end of resistance 29. Under normal conditions, therefore, the anode of diode 17 is more positive than the cathode, establishing current flow through the voltage divider. The relative values of voltage dividing resistances 29 and 30 is such that the major portion of the voltage drop takes place across resistance 29 so that the junction of the resistances and hence the base of transistor 18 is only slightly more positive than the negative bus 27 and the emitter 22 so that transistor 18 is in a slightly conductive condition.

Diode 17, which forms part of the voltage dividing network for transistor 18, limits the amplitude of the positive pulse which is applied to the base of transistor 18 thereby preventing that transistor from being overdriven. That is, since the value of resistance 29 is normally quite large compared to the resistance of diode 17 in its conducting state, diode 17 may be considered as a switch element which passes positive input pulse 13 to the base of transistor 18 as long as the amplitude of pulse 13 is less than the voltage drop across resistance 30. That is, the cathode of diode 17 is normally more positive than the negative bus 26 by an amount determined by the relative values of resistances 29 and 30. The anode of diode 17 is more positive than the cathode causing the diode to conduct. A positive pulse 13 causes the current through diode 17 to decrease. This increases the voltage at base 20 of transistor 18, causing it to conduct more heavily. If the amplitude of the pulse is large enough so that no current flows through the diode then the diode becomes nonconducting, thereby limiting the degree of conduction of transistor 18.

The positive pulse applied to the base of transistor 18 causes the transistor to become heavily conducting. The increased voltage drop across collector resistance 24 produces an amplified but phase inverted pulse 32 which is applied through a suitable coupling capacitor 33 to a mono-stable or one-shot multivibrator shown generally at 34. One-shot multivibrator 34 includes a'pair of NPN transistors 35 and 36 which are so interconnected that transistor 35 is normally conducting and transistor 36 is normally nonconducting. The appearance of negative pulse 32 from pulse amplifier 19 reverses the conductive conditions of these two transistors causing transistor 35 to become nonconducting and transistor 36 to become conducting for a fixed interval of time determined by the time constant of the multivibrator circuit. There is therefore produced at the output lead 37 of one-shot multivibrator 34 a negative going pulse 38 of fixed duration, amplitude and polarity, which pulse is applied to the R.F. amplifying stages of the receiver signal transmission path to blank the receiver upon occurrence of the detected noise pulse from the noise blanking channel.

Transistor 35 of the one-shot multivibrator 34 includes a base 39, an emitter 40 and a collector 41, and transistor 36 similarly includes a base 42, an emitter 43 and a collector 44. Emitter 40 of transistor 35 is connected directly to negative bus 26, the collector is connected to positive bus 23 through a suitable collector resistor 45, and base 39 is connected to the positive bus 23 through a positive base return resistor 46. The collector of transistor 35 is directly coupled to the base 42 of transistor 36 through a lead 47 and collector 44 of transistor 36 is connected to the base of transistor 35 through the coupling capacitance 48. Emitter 43 of transistor 36 is connected to the negative bus through a bypassed emitter resistance 49 and the collector is connected to the positive bus through an unbypassed collector resistance 50.

Under normal conditions, transistor 35 is conducting by virtue of the positive base return through resistance 46. That is, base electrode 39 is connected to the positive bus 23 through resistance 46 and is therefore substantially at the positive potential of that bus. Since emitter electrode 40 is connected directly to negative bus 26, the full supply voltage appears across the emitter-base junction of the transistor and transistor 35 is biased into saturation. With transistor 35 in the saturated condition, the collectoremitter resistance of the transistor is very low and collector 41 of transistor 35 is essentially at the potential of the negative bus 26. Collector 41 is connected directly to base 42 of transistor 36 and, therefore, holds base 42 slightly more negative than emitter 43 of the transistor. As a result, the base-emitter junction of transistor 36 is reverse biased and transistor 36 is biased into cut-off. Upon the appearance of input negative pulse 32 in response to detected pulse 13 at teminals 14 and 15, base 39 of transistor 35 is suddenly driven negative, biasing transistor 35 to cut-oft. The potential at collector 41 rises to the value of the voltage at positive bus 23 and base electrode 42 of transistor 36 is therefore raised to this positive potential driving transistor 36 into saturation.

When transistor 36 conducts, its emitter-collector resistance drops to a very low value and collector 44 of transistor 36 goes essentially to the potential at the negative bus 26. Hence, the charge on coupling capacitor 48 drags the voltage at base 39 down below the potential at negative bus 26 by an amount E equal to the supply voltage. Similarly, coupling capacitor 52 is also forced negative with respect to bus 26 by virtue of the change on the capacitor. During the period that transistor 36 is conducting, base 39 of transistor 35 is rising from E volts with respect to bus 26 to the positive potential on bus 23 as capacitor 48 discharges through resistance 46. When the voltage at base 39 becomes more positive than the voltage at bus 26 the conducting states of transistors 35 and 36 reverse again and return to the normal condition. The duration of the time interval during which transistor a 36 is conducting and hence the interval of negative pulse 38 is determined by the R-C time constant of capacitor 48 and resistance 46. The circuit then remains in its normal state, i.e., with transistor 35 conducting and transistor 36 nonconducting, until another input triggering pulse from pulse amplifier 28 appears.

Also coupled to the one-shot multivibrator is a feedback loop shown generally at 51 which produces a control voltage which is a function of the repetition rate of these gating pulses. That is, each triggering of one-shot multivibrator 34, which produces a blanking pulse 38, also produces a control voltage which is fed back to pulse amplifier 20. A further coupling capacitor 52 is connected in series with leakage resistance 53 between collector 44 and negative bus 26. A diode element 54 is connected at the junction of resistance 53 and coupling capacitor 52. Diode ais so poled as to pass only negative pulses to charge a storage capacitor 55 to a polarity which is negative with respect to bus 26. As was pointed out previously, when ever transistor 36 is driven to the conducting state upon the appearance of the triggering pulse 32, the charges on coupling capacitors 48 and 52 drive the base of transistor 35 and the junction of diode 54 and coupling capacitor 52 to a value of E volts with respect to negative bus 26. Storage capacitor 55 therefore tends to charge towards this value E volts which is negative with respect to bus 26. The voltage to which storage capacitor 55 charges during each blanking pulse interval 38 is determined by the relative values of capacitors 52 and 55. Since capacitor 55 is normally quite large relative to capacitor 52, on the order of ten to one or greater, capacitor 55 only charges to a fraction of the voltage E on capacitor 52. On successive pulses, capacitor 55 charges further thereby increasing the voltage in a stepwise fashion. In the interval between blanking pulses 38, storage capacitor 55 discharges toward the voltage on bus 23 through series resistances 56 and 29.

Resistances 29 and 56 which form the discharge path for storage capacitor 55 have a substantial R-C time constant so that capacitor 55 is only partially discharged between blanking pulses with the degree of discharge depending on the interval between the pulses. The average control voltage level on capacitor 55 is, therefore, proportional to the interval between pulses, and hence to the repetition rate of blanking pulses 38. Base 20 of transis tor i8 is connected to capacitor 55 and the control voltage biases transistor 18 to disable the. pulse amplifier whenever the repetition rate exceeds a predetermined value. As long as the repetition rate of theblanking pulses does not exceed this predetermined value, capacitor 55 discharges sufficiently between pulses so that the negative voltage on capacitor 55 is insufficient to bias transistor 18 into the nonconducting state. If, however, the repetition rate of the detected noise pulses impressed on input terminals 14 and 15, and hence blanking pulses 38, exceeds this predetermined value, the average voltage level on storage capacitor 55 increases with successive pulses until the negative bias applied to transistor 18 is sufiicient to reverse bias the emitter-base junction causing transistor 18 to become nonconducting. The negative control voltage on capacitor 55 is also impressed on the anode of diode switch 17, reverse biasing the diode and insuring that any positive detected pulses 13 impressed on input terminals 14 and 15 are not transmitted to base electrode 20. This provides additional assurance that a noise pulse of very high amplitude does not inadvertently drive amplifier into conduction, thereby producing additional blanking pulses during the undesired interval.

The operation of the blanking switch and switch control arrangement illustrated in FIG. 2 may be understood most clearly in view of the following discussion. Upon the appearance of a positive pulse from the pulse detector of the blanking channel at input terminals 14 and 15, base 20 of NPN transistor 18 of pulse amplifier 20, which is biased slightly positive, is driven highly positive causing the transistor to conduct heavily. The heavy conduction of transistor 18 produces a sufficient voltage drop across collector resistance 24 to produce negative pulse 32 which is transmitted through coupling capacitor 33 directly to base 39 of transistor 35 of the one-shot multivibrator. Transistor 35 which is normally in saturation is driven into cut-off by negative going pulse 32. The voltage at collector 41 of transistor 35 rises from the voltage at negative bus 26 to the voltage at positive bus 23, driving normally nonconducting transistor 36 into saturation. This produces a negative blanking pulse 38 of fixed amplitude at collector 44 of transistor 36. Negative blanking pulse 38 is applied over output lead 37 to one or more of the RF. amplifier stages in the main signal transmission path of the receiver, driving the amplifiers into the nonconducting state thereby preventing transmission of the noise pulse to the remainder of the receiver.

The negative noise blanking pulses 38 charge capacitor negative with respect to bus 26 through coupling capacitor 52 and diode 54. That is, during the time that transistor 36 is in the nonconducting state coupling capacitor 52 charges essentially to the value of the supply voltage at bus 23 with the polarity indicated on the drawing. When transistor 36 is driven into saturation in response to negative pulse 32, emitter 44 is driven essentially to the potential on negative bus 26. Since coupling capacitor 52 cannot discharge instantaneously the junction of capacitor 52 and diode 54 is instantaneously driven negative relative to bus 26 by an amount equal to the voltage on capacitor 52. Diode 54 then conducts and the charge on capacitor 52 is rapidly transferred to control capacitor 55 since the forward resistance of diode 54 is very low. As has been pointed out previously, since coupling capacitor 52 is substantially smaller than capacitor 55, the charge stored in capacitor 52 is not sufficient to charge capacitor 55 to the full value of the blanking pulse amplitude which is substantially the supply voltage. After approximately ten microseconds, the duration of blanking pulse 38, transistors 35 and 36 revert to their original condition with transistor 35 being driven into saturation and transistor 36 into the nonconducting state. During this interval, capacitor 55 begins discharging through resistors 56 and 29 to the voltage level on positive bus 23. Discharge of capacitor 55 continues until the appearance of the next detected pulse 13 at which time the transistors of one-shot multivibrator 31 again reverse their conducting states and supply a negative blanking pulse 38' to capacitor 55 to charge the capacitor. As capacitor 55 has not discharged completely, the voltage on capacitor 55 increases by a small increment. It will be apparent that the average level maintained on capacitor 55 is therefore a function of the interval between the blanking pulses and hence is proportional to the repetition rate of the noise pulses. That is, as the time interval between pulses decreases because the repetition rate of the noise pulses has increased, the average control voltage level increases since the degree to which capacitor 55 has discharged during the interval between pulses has been reduced. If the repetition rate is greater than the predetermined critical rate, the discharge between pulses is reduced sufficiently so that after N number of pulses, a number determined by the actual values of the circuit components utilized, the voltage on capacitor 55 has increased sufiiciently so that the negative bias applied to base 20 of NPN transistor 18 reverse biases the baseemitter junction of the transistor causing it to become nonconducting and preventing succeeding noise pulses 13 from triggering multivibrator 34.

After capacitor 55 is charged, no pulses goes to oneshot multivibrator 34 so capacitor 55 begins to discharge. After a certain time, determined by the time constant of the circuit, capacitor 55 has discharged sufficiently to permit transistor 18 to conduct and pulses again operate 9 the one-shot multivibrator. If the repetition rate is still too high, capacitor 55 is again charged and the amplifier 20 is turned off as described previously. Sampling of the pulse conditions in this manner continues until the pulse repetition rate falls below the critical value and normal operation is restored.

The following component values have been utilized in a circuit constructed in accordance with FIG. 2, and although these component values are not to be considered as limiting, a circuit utilizing these values provided satisfactory operation:

Capacitor 16 .001 microfarads. Diode 17 1N198. Transistor 18 2N706.

Resistor 24 3300 ohms. Resistor 25 1 kiloohm. Resistor 29 68 kiloohms. Resistor 30 4700 ohms. Capacitor 33 .001 microfarads. Transistor 35 2N706. Transistor 36 2N706.

Resistor 45 3300 ohms. Resistor 46 18 kiloohms. Capacitor 48 680 picofarads. Resistor 49 120 ohms. Resistor 50 2 kiloohms. Capacitor 52 .001 microfarads. Resistor 53 1 kiloohm. Diode 54 1N198. Capacitor 55 01. microfarad. Resistor 56 kiloohms.

FIG. 3 illustrates an alternative construction of the blanking switch and switch control circuit of FIG. 2 wherein electron discharge devices, such as vacuum tubes, are utilized in place of the semiconductor transistor devices. FIG. 3 operates essentially in the same manner as does the solid state circuit of FIG. 2 and includes a pulse amplifier 60 for amplifying noise pulses, a monostable or one-shot multivibrator 61, operative in response to the amplified noise pulses from amplifier 60 to produce the blanking pulses of suitable polarity, duration and amplitude, and a switch control feedback loop 62 for producing a control voltage proportional to the repetition rate of the blanking pulses for disabling the pulse amplifier and switch arrangement in the event that the repetition rate of the detected noise pulses rises above a predetermined value.

Amplifier 60 includes a vacuum triode element 63 having a cathode 64, a control grid 65, and an anode 66. Anode 66 is connected to the positive terminal B-lof a suitable source of energizing voltage through a suitable anode resistance 67, and. cathode .64 is connected to a point of reference potential such as ground through a suitable cathode dropping resistance 68 which is bypassed for AG by a suitable capacitor 69. Vacuum triode 63 of pulse amplifier 60 is biased to operate as a Class AB pulse amplifier by means of a biasing network comprising the voltage divider resistances 70 and 71 which are intercoupled through the diode switch 72. The biasing network 70, 71 and 72 functions in the same manner as the network illustrated in FIG. 2 and applies a slightly positive going voltage to control grid 65 of tube 63 to maintain the tube slightly conducting. Positive detected pulses 13 are impressed on input terminal 73 and coupled to the control grid through a coupling capacitor 74 and the diode switch 72. The appearance of the input noise pulses 13 drives vacuum triode 63 into heavy conduction, producing a negative going pulse 75 at the anodes 66 of substantially greater amplitude than input pulses 13. Negative pulse 75 is coupled through a suitable coupling capacitor 77 to one-shot multivibrator 61 causing it to reverse its conducting state and producing a negative blanking pulse 78 of fixed duration at output terminals 79.

Switch 61 is a monostable or one-shot multivibrator with positive grid return and includes a pair of crosscoupled vacuum triodes 80 and 81. Vacuum triodes 80 and 81 each include cathodes 82 and 83, control grids S4 and 85, and anodes 86 and 87. The anodes of triodes 80 and 81 are connected respectively to the positive terminal 13-}- of the power supply through anode resistors 88 and 89. Anode 86 is directly coupled to the control grid of triode 81 and anode 87 of triode 81 is coupled to the control grid 84 of triode 80 through a suitable coupling capacitance 91. Control grid 34 of triode 80 is returned to the positive B+ terminal through a resistance 512.

In the stable condition, triode S0 is conducting since its control grid 84 is held slightly positive with respect to its cathode by the positive grid return through resistance 92. The voltage at anode 86 is, therefore, quite low, because of the voltage drop due to the anode current flowing in resistance 8?, thereby biasing control grid 85 to cut-oil. Negative triggering pulse 75 is applied to grid 84 and decreases the anode current in triode 80 causing a corresponding rise in the anode voltage. The rise in anode voltage is coupled to the control grid 85 of triode 81. Anode current begins to How in triode 81 producing a voltage drop across anode resistance 39, therebydecreasing the voltage at anode 87. Capacitor 91 couples this decrease in anode voltage to control grid 84 of triode 80 further decreasing the anode current through triode 80 and increasing the voltage at its anode 86. This action continues until triode 80 is cut off and triode 81 is conducting heavily. The circuit remains in this condition as long as discharge of coupling capacitor 91 maintains sufficient negative potential on control grid 84 of triode 80 to keep triode 80 in the nonconducting state; When capacitor 91 has discharged sufficiently to allow control grid 84 to rise to cut-off, triode 80 conducts and the potential at anode 86 decreases. This decrease is, of course, coupled directly to the control grid 85 of triode 81 causing the anode current in that triode to decrease and the plate potential to rise. This action continues at a very rapid rate until triode 80 is driven into the heavily conducting state and triode 81 is driven into the nonconducting state. The multivibrator remains in this condition until the appearance of the next negative triggering pulse 75 from the pulse amplifier 60. The negative output pulses appearing at anode 87 of triode 81 are coupled from the output terminals 79 to one of the RF. amplifier stages of the main signal transmission path of the receiver to blank the receiver and prevent passage of the noise pulse.

The negative noise pulse 78 is also transmitted over the control voltage feedback path 62 to produce a control voltage proportional to the pulse repetition rate which disables pulse amplifier 60 whenever the repetition rate increases beyond the predetermined level. To this end, the negative noise blanking pulses 78 are coupled through a coupling capacitor 92 to a diode 93 which is poled so as to pass only negative pulses. The negative pulses transmitted by diode 93 discharge a storage capacitor 94 to produce a control voltage, the average level of which is proportional to the repetition rate of the blanking pulses 78, and hence to the detected noise pulses. In a manner similar to that described with reference to the circuit of FIG. 2, the control voltage biases amplifier 60 into cut-ofi whenever the repetition rate exceeds a predetermined value, thereby disabling the noise blanking channel of the receiver.

Although the circuit arrangement described above is effective to disable the noise blanking channel whenever the noise impulse rate is sufficiently high to cause excessive blanking of the receiver, the system is flexible enough to permit blanking of a burst of closely spaced noise impulses provided that the burst is of sufliciently short duration. That is, since capacitor 55 takes a finite period of time to charge up to a value of voltage such that pulse amplifier 18 is biased into cut-off, a burst of closely spaced noise impulses will cause the capacitor to charge rapidly towards this critical voltage level. If the burst of impulses is of short duration and terminates before the voltage rises sufliciently, the disabling circuit is not actuated. Thus even though the impulse rate may have been very high over this interval, if the interval is short enough the disabling circuit is not actuated. If on the other hand the interval of closely spaced impulses is greater than a predetermined duration (a duration which is determined by the R-C charging and discharging times) the blanking circuit is disabled. It is, therefore, clear that the present circuit arrangement is one of great flexibility since it will permit the blanking circuit to blank out noise impulses at a very high rate for a very short time but will not permit such a high blanking rate on anything approaching a steady state basis.

It will be apparent from the foregoing description that a new and novel communication receiver has been described which is particularly useful in mobile radio service and which includes a simple, effective circuit arrangement for disabling the noise blanking channel of the receiver whenever the blanking rate, due to any cause, becomes excessive.

While a number of particular embodiments of this invention have been shown, it will, of course, be understood that the invention is not limited thereto since many modifications, both in the circuit arrangements and in the instrumentalities employed, may be made. It is contemplated by the appended claims to cover any such modifications as fall within the true spirit and scope of this invention.

What is claimed as new and desired to be secured by Letters Patent is:

1. In a communication receiver the combination comprising a signal transmission path including a plurality of stages for amplifying, converting and detecting the received signal and for reproducing the intelligence, circuit means for interrupting transmission in said signal transmission path to prevent transmission of noise impulses including means to detect noise impulses, means responsive to said detected impulses for producing blanking pulses and for impressing said pulses on one of the stages in said path to prevent transmission of the signal and noise impulses, and means for disabling said interrupting means whenever the rate at which said transmission path is interrupted exceeds a predetermined rate, said last named means including means to produce a control signal proportional to the rate at which said channel is interrupted, and means to apply said control signal to said interrupting circuit to terminate production of said blanking pulses when said control signal exceeds a predetermined value.

2. In a communication receiver the combination comprising a signal transmission path including a plurality of stages for amplifying, converting and detecting the received signal and for reproducing the detected intelligence, a noise blanking channel for interrupting transmission in said signal transmission path to prevent transmission of noise impulses including means to detect noise impulses, means responsive to said detected noise impulses for producing blanking pulse, means coupling said blanking pulses to a selected stage of said signal transmission path to prevent transmission of noise impulses, control means coupled to said blanking channel for disabling said channel whenever the repetition rate of said noise impulses and of said blanking pulses produced in response thereto exceeds a predetermined rate, including means to produce a unidirectional control signal proportional to the blanking pulse rate, and means coupling said control signal to said path to disable said channel when said control signal exceeds a predetermined value.

3. The communication receiver according to claim 2 wherein said disabling means includes a storage capacitor, a unidirectional conducting element for charging said T12 capacitor in response to said blanking pulses to produce said control signal.

4. In a communication receiver the combination comprising a signal transmission path including a plurality of stages for amplifying, converting, and detecting the received signal and for reproducing the detected intelligence, a noise blanking channel for interrupting transmission in said signal transmission path to prevent transmission of noise impulses including means to detect noise impulses, means responsive to said detected noise impulses for producing blanking pulses, including a monostable multivibrator operative to produce a blanking pulse in response to each detected pulse, means for coupling said blanking pulses to a selected stage of said transmission path to disable said stage and prevent further transmission of noise impulses, a control circuit responsive to said blanking pulses to disable said blanking channel whenever the blanking pulse rate exceeds a predetermined rate including a storage capacitor, unidirectional conducting means coupled to said multivibrator for charging said capacitor in response to said blanking pulses and producing a control voltage proportional to said blanking pulse rate, means coupling said capacitor to said blanking channel for biasing said channel into an inoperative state to prevent production of further blanking pulses whenever said control voltage exceeds a predetermined level.

5. A communication receiver according to claim 4 wherein said control circuit includes a discharge path for said capacitor having a greater time constant than the charging path whereby said capacitor discharges and becomes inoperative if the repetition rate of said noise impulses decreases below said predetermined rate.

6. In a communication receiver according to claim 4 wherein said control circuit includes a pulse amplifier for amplifying said detected pulses and triggering said multivibrator, means coupling said storage capacitor in said control circuit to said pulse amplifier to bias said amplifier into nonconduction whenever said control voltage exceeds a predetermined level thereby terminating further triggering pulses to said multivibrator.

7. In a communication receiver the combination comprising a signal transmission path including a plurality of stages for amplifying, converting, and detecting the received signal and for reproducing the detected intelligence, a noise blanking channel for interrupting transmission in said signal transmission path to prevent transmission of noise impulses including means to detect noise impulses, means responsive to said detected noise impulses for producing blanking pulses including a pulse amplifier, a monostable multivibrator to produce a blanking pulse in response to each detected pulse amplified by said pulse amplifier, means for simultaneously coupling said blanking pulses to a selected stage of said transmission path and to the input of a control circuit, said control circuit including a storage capacitor, a charging path for said capacitor to produce a control voltage proportional to the blanking pulse repetition rate, means coupling said capacitor to said pulse amplifier to bias it into cut-01f when said control voltage exceeds a predetermined voltage thereby terminating further production of blanking pulses.

8. In a communication receiver the combination comprising a signal transmission path including radio-frequency amplifying stages for amplifying the received radio frequency signals, at least one frequency converting stage for converting the received signal to a lower frequency, detecting means for retrieving the intelligence from said signal and means for reproducing the intelligence at the output of the receiver, a noise blanking channel for interrupting transmission in the radio frequency amplifying stages to prevent transmission of noise impulses including means to detect noise impulses, means responsive to said detected noise impulses for producing blanking pulses, means coupling said blanking pulses to 13 a selected stage of said noise transmission path to prevent transmission of noise impulses, control means coupled to said blanking channel for disabling said channel whenever the repetition rate of said noise impulses and of said blanking pulses produced in response thereto exceeds a predetermined rate, including means to produce a unidirectional control signal proportional to the blanking pulse rate, and means for coupling said control signal to one of the radio amplifying stages in said path to disable said channel when said control signal exceeds a pre- 10 determined value.

9. A communication receiver according to claim 8 1 wherein said control means includes a storage capacitor, a unidirectional conducting means coupled to said capacitor for passing current during said blanking pulses to charge said capacitor and produce a control voltage pro- 5 portional to the repetition rate of said pulses.

References Cited in the file of this patent UNITED STATES PATENTS 2,365,583 Nagel et a1 Dec. 19, 1944 2,987,632 Milford June 6, 1961 3,014,127 Vlasak Dec. 19, 1961 

1. IN A COMMUNICATION RECEIVER THE COMBINATION COMPRISING A SIGNAL TRANSMISSION PATH INCLUDING A PLURALITY OF STAGES FOR AMPLIFYING, CONVERTING AND DETECTING THE RECEIVED SIGNAL AND FOR REPRODUCING THE INTELLIGENCE, CIRCUIT MEANS FOR INTERRUPTING TRANSMISSION IN SAID SIGNAL TRANSMISSION PATH TO PREVENT TRANSMISSION OF NOISE IMPULSES INCLUDING MEANS TO DETECT NOISE IMPULSES, MEANS RESPONSIVE TO SAID DETECTED IMPULSES FOR PRODUCING BLANKING PULSES AND FOR IMPRESSING SAID PULSES ON ONE OF THE STAGES IN SAID PATH TO PREVENT TRANSMISSION OF THE SIGNAL AND NOISE IMPULSES, AND MEANS FOR DISABLING SAID INTERRUPTING MEANS WHENEVER THE RATE AT WHICH SAID TRANSMISSION PATH IS INTERRUPTED EXCEEDS A PREDETERMINED RATE, SAID LAST NAMED MEANS INCLUDING MEANS TO PRODUCE A CONTROL SIGNAL PROPORTIONAL TO THE RATE AT WHICH SAID CHANNEL IS INTERRUPTED, AND MEANS TO APPLY SAID CONTROL SIGNAL TO SAID INTERRUPTING CIRCUIT TO TERMINATE PRODUCTION OF SAID BLANK PULSES WHEN SAID CONTROL SIGNAL EXCEEDS A PREDETERMINED VALUE. 